This mechanism relates to control mechanisms for drive trains of vehicles and more particularly relates to a bypass mechanism operable by the vehicle driver to route fuel around an electronically controlled fuel injection system.
The U.S. Army uses a fuel injection device known as an HMU, or hydromechanical unit, on its M1 and M1A1 Tanks. The HMU is an assembly of electronically actuatable valves and other components which are controlled by an ECU (electronic control unit). To prevent engine damage the ECU is programmed to initiate several protective modes and reduces fuel flow from the HMU if certain performance irregularities occur in the engine or its control systems. In one of the protective modes, referred to as "protective mode III", the flow rate of fuel to the engine is reduced to about 120 lbs/hour, so the engine generates only about 72 hp, or 5 percent of its capacity. In this condition the tank is limited to a maximum speed of 1 mph.
In a battle or emergency scenario, it may be necessary to drive the tank faster than protective mode III permits. Further. it is possible that battle damage could cut off electrical power to the ECU and thereby adversely limit fuel flow to the engine.
To address these difficulties, we have created a fuel bypass device which can route additional fuel to the engine during protective mode III or during failure of the ECU. Our bypass device is independent of the ECU and comprises a modular add-on unit which can be retrofitted on tanks without disassembly of HMUs or modification of ECU logic. The device includes a set of fixed-oriface valves connected in parallel with one another, this set of valves being operable to effect stepwise increases or decreases of fuel flow to the engine. The control circuitry for the solenoid-actuated valves includes a means to shut them off automatically when the transmission is moved out of the "drive" or "reverse" gear range. The automatic shut off is advantageous because potential damage can be caused by over-revving of the tank's engine if it receives too much fuel while not under load. An absence of load on the engine occurs, for example, when the automatic transmission is in the "neutral" gear range.